Recent work from our laboratories have demonstrated the choline supplementation given to rats during both pre- and postnatal development produces long-term modification of biochemical and neuroanatomical markers of basal forebrain cholinergic function and long-term facilitation of visuospatial memory as indexed by choice accuracy on a radial maze. Choline supplementation to male rats produces these organizational changes in visuospatial memory only when given during embryonic days 12-17 or postnatal days 16-30 but not during postnatal days 1-15 when several potentially important developmental events are occurring in the rat. The major goal of this proposal is to further evaluate these multiple sensitive periods of choline effectiveness. Two hypotheses will be tested. 1. The sexual differentiation hypothesis predicts that exposure to gonadal steroid hormones during postnatal days 1-15 is required for choline to exert its effects on postnatal days 15-30. Preliminary data demonstrating that female rats are only sensitive to choline action when it is administered prenatally support this hypothesis. Two lines of research are planned to examine this phenomenon. Gonadal hormone exposure during early postnatal development will be manipulated and subjects will be tested as adults for their visuospatial memory using a radial arm maze task with both working and reference memory comment of spatial memory using a water maze navigational task. 2. The choline saturation hypothesis suggests that pups may be receiving exceptionally high levels of choline from the dams milk during postnatal day 1-15, thus supplemental choline may not be able to enhance brain choline exposure during this period. This hypothesis will be tested by raising pups with mothers of different lactational states in order to vary their exposure to choline naturally occurring in the dam's milk. For all these studies, subjects will be exposed to choline during one of 4 periods of development (Prenatal days 12-17, postnatal days 1-15, 16-30, or prenatal day 12-postnatal day 30) or not exposed to choline. These data should provide us with information about the development, hormonal modulation, and natural nutritional control of choline's organizational facilitation of visuospatial memory.